This invention relates to a method for the distribution of data, e.g. video signals, and in particular to the distribution of video signals into a residential or commercial building, for example as part of a video-on-demand system.
With the trend towards deregulation in the industry and with the development of new technologies, television programs and video are obtainable through increasing number of means. No longer is a viewer restricted to the choice of programming provided by conventional terrestrial broadcasters, supplemented by the purchase of rental of prerecorded video tapes, laser discs, video CDs and others. Television and video programming are now available by satellite and also cable systems. In addition, the content of such systems has broadened beyond conventional programming to include multi-media elements, Internet connections, interactive television and so on.
One difficulty with such alternative channels is distribution of television programs and the like. With a conventional cable system, appropriate cabling is provided directly to a consumer""s premises. However, with the advent of digital technology in telecommunications, it is becoming increasingly popular for television and video programming to be provided by telecommunications companies through existing telephone networks.
Generally in densely populated urban cities it is common for telecommunication services to be provided through a network of optical fibre cables that extend as far as the basement of a residential or commercial building, and then for signals to be carried to individual users within a building by a twisted-pair cable. However, while a twisted-pair cable is more than adequate for carrying voice transmissions and simple data transmission (e.g. fax or Internet transmissions), such a cable introduce channel distortion that degrades picture quality.
There are a large number of existing patents in this general field of technology and of which the following are a representative sample. U.S. Pat. No. 5,541,757 describes a typical video-on-demand (VOD) system that uses fibre cables for video distribution. Using fibre cables the system can achieve high-quality results, but the fibre cables are expensive themselves and must be installed in the premises concerned which increases both the cost and inconvenience of the installation of the system. U.S. Pat. No. 5,592,482 and U.S. Pat. No. 5,625,863 describe methods for distributing analog video signal using in-wall electrical wiring. The systems of these patents, however, are limited to the transmission of analog signals, and furthermore the use of conventional electrical wiring in this context can be unsafe.
U.S. Pat. No. 5,410,343, U.S. Pat. No. 5,247,347, U.S. Pat. No. 5,621,455 and U.S. Pat. No. 5,594,491 describe VOD systems for distributing MPEG-1 format videos (MPEG being an acronym for the Motion Picture Experts Group) over public switched telephone networks using Asymmetric Digital Subscriber Line (ADSL) technology. ADSL is a transmission technology for providing high data rate service from a central office to subscribers. The ADSL Forum has adopted multitone for the ADSL-2 standard based on the result of simulations and measurements from existing loop plants up to 18 kft. However, it is not normally necessary to transport data beyond a few kilo-feet in a densely populated city, and in addition private VOD systems, intranets and multimedia information systems for hotels, large corporations, hospitals and so on generally only require high bandwidth transmission for only a few hundred feet. Thus while providing high-quality service, such proposed ADSL systems are unnecessarily expensive.
An increasingly common possibility, especially for VOD and the like services provided by telecommunications companies, is a so-called xe2x80x9cfibre-to-the-curbxe2x80x9d system as shown in FIG. 1 in which fibre cables are laid to the basement of a residential or commercial building, and then conventional twisted pairs are used for in-building distribution. A disadvantage with this technique, however, is the problem of channel distortion in the relatively low-quality twisted-pairs. One conventional way of overcoming such problems is to provide channel equalization at the receiver, for example using a technique of the type outlined in U.S. Pat. No. 5,461,640. One disadvantage with using equalisation at the receiver, however, is that an equalizer must be provided for every subscriber.
It is an object of the invention to overcome the problem of channel distortion in a cost-effective manner to allow the use of twisted pairs for carrying high-rate data transmissions in a building.
According to the present invention there is provided a data transmission system comprising,
a server array,
a plurality of subscribers,
a plurality of data lines from said server,
connection means for connecting said subscribers to said server, said subscribers being connected to said connection means by respective cables, and
pre-emphasis filter means provided in each said data line from said server.
The present invention is applicable in any situation where the cables has relatively linear distortion properties. In some cases, for example data distribution at high rates such as 100 Mbps, even coaxial cables may suffer such distortion. However the present invention is particularly applicable to systems using twisted-pair cables which suffer such distortion at lower rates (e.g. less than 10 Mbps) at which MPEG-2 videos are transmitted.
In order to establish the correct properties of the pre-emphasis filter required to compensate for the distortion, the data transmission system further comprises means for characterizing the distortion properties of each channel from said server to a said subscriber, and means for calculating and establishing the required properties of said pre-emphasis filter means to compensate for said distortion properties.
Preferably the characterization means comprises means for transmitting a test signal to a receiver at a said subscriber, means for returning said test signal from said receiver to said transmitting means, and means for calculating the distortion properties of a said channel from the delay in returning said signal from said receiver to said transmitting means.
In a particularly preferred embodiment the transmitting means is adapted to generate an increasing test signal preceded by a header, and said receiving means is adapted to generate a first return signal when said header is received and a second return signal when said test signal reaches a predetermined threshold at said receiving means, and said transmitting means includes clock means for determining the time interval between the test signal being transmitted by said transmitting means and the return signals being received at said transmitting means from said receiving means.
The test signal is preferably a linearly increasing signal which may be quantized or non-quantized.
Preferably the pre-emphasis filter is a tap-line filter, preferably either a conventional tap-line filter or a fractional tap-line filter. The best results are obtained with a fractional tap-line filter which can obtain good compensation for the channel distortion with as little as a third order filter. However, other forms of pre-emphasis filter may also be used, such as finite impulse response filters whose coefficients can be obtained easily by using an inverse fast Fourier transform.
Viewed from another aspect the present invention provides apparatus for the characterization of the amplitude and phase response of a data channel between a transmitter and a receiver, wherein said transmitter comprises means for transmitting a test signal to said receiver, means for detecting the arrival of a return signal from said receiver and means for timing the delay between transmitting said test signal and detecting said return signal, wherein said receiver comprises means for generating a return signal when a received transmitted signal exceeds a predetermined threshold, and means for calculating the amplitude and phase response of said data line from said delay.
In a preferred embodiment the transmitting means is adapted to generate an increasing test signal preceded by a header, and said receiving means is adapted to generate a first return signal when said header is received and a second return signal when said test signal reaches a predetermined threshold at said receiving means, and said transmitting means includes clock means for determining the time interval between aid test signal being transmitted by said transmitting means and said return signals being received at said transmitting means from said receiving means.
Again the test signal is preferably a quantized or non-quantized linearly increasing signal.
Viewed from a still further aspect the present invention provides a method for transmitting data from a server to a plurality of subscribers along a data channel comprising a cable having relatively linear distortion properties, comprising characterizing the distortion properties of said data channel prior to transmitting said data along said data channel, and passing said data through a pre-emphasis filter prior to transmission along said data channel to compensate for said distortion.
Viewed from a still further aspect the present invention provides a method for the characterization of the amplitude and phase response of a data channel between a transmitter and a receiver, comprising transmitting a test signal to said receiver, generating a return signal from said receiver to said transmitter when a received transmitted signal exceeds a predetermined threshold, detecting the arrival at said transmitter of a said return signal from said receiver and timing the delay between transmitting said test signal and detecting said return signal, and calculating the amplitude and phase response from said delay.